OSSEC alerts without hosting SMTP

Find the Postfix main.cf file and set:

smtpd_tls_security_level = may

I have no idea where main.cf might be located in CentOS with Plesk, but the usual location is /etc/postfix/main.cf.

If TLS is being enforced via the Postfix config, smtpd_tls_security_level will be set to encrypt. may makes it optional.

http://www.postfix.org/TLS_README.html

• Can I actually send web mail without having a registered domain name?

Yes. You do need to send with a valid e-mail address as sender, though. Your best solution (if you do want to send e-mail) is to use an address and the relay server for that address. For example, use a gmail.com address and use an authenticated TLS port 587 connection to gmail to send your mail, even if the destination is not at gmail.

• Without a FQDN, what should i use for "/etc/mailname" and the "hostname" in main.cf?

if you have a fixed IP address, you should have a FQDN, but as long as you use the solution above, it doesn't really matter.

• Are there any articles or resources on postfix that you think would be applicable to my situation?

If you choose a gmail account to use for this, search for articles on setting up postfix to relay to gmail. My first result on google was http://mhawthorne.net/posts/postfix-configuring-gmail-as-relay.html which seems concise and to the point. For other e-mail providers you should be able to find similar documentation.

I said above "if you do want to send e-mail". If you are configuring alerts, you way want to investigate free sms services as well.